1. Field of the Invention
The present invention relates to a laser diode, a semiconductor light-emitting device, and a method of production thereof, more particularly relates to a laser diode for self pulsation, a semiconductor light-emitting device including a plurality of laser diode elements including the laser diode, and a method of producing the laser diode.
2. Description of the Related Art
Generally, an optical pickup is built in an optical disk apparatus for reading (reproducing) information recorded on an optical recording medium (hereinafter also referred to as an optical disk) optically recording information such as a CD (compact disk), DVD (digital versatile disk), and MD (mini disk) and writing (storing) information thereon.
A laser diode is used for a light source of the above optical disk apparatus and optical pickup.
As the wavelength of the light source, when the types of optical disks (optical disk systems) differ, laser lights of different wavelengths are used. For example, laser light having a wavelength of 780 nm is used for playing back CDs and laser light having a wavelength of 650 nm is used for playing back DVDs.
In recent years, a self pulsation type laser diode has been developed to reduce noise according to returned light of laser light in the laser diode of the 650 nm band for DVDs as above.
A self pulsation type laser diode has the advantage of preventing emission noise caused by high frequency superimposition during laser driving as in the prior art.
As an example of the above self pulsation type laser diode, a structure having a saturatable absorption layer has been developed (refer to Preprints of 47th Joint Presentations of Applied Physics, page 1147, lecture 29a-N-2).
FIG. 12 is a cross-sectional view of the above laser diode.
An n-type buffer layer 51 comprised for example of InGaP, an n-type clad layer 52 comprised for example of AlGaInP, an active layer (multiquantum well structure having an oscillation wavelength of 650 nm) 53, and a p-type clad layer 54 comprised for example of AlGaInP are stacked on an n-type substrate 50 comprised for example of GaAs.
A saturatable absorption layer doped at high concentration 55 is formed at the upper layer of the p-type clad layer 54.
Further at the upper layer of that is formed a p-type clad layer 56 comprised for example of AlGaInP so as to protrude out in a current injection stripe region. An n-type block layer 57 made by AlInP is formed at regions except for the current injection stripe region. An n-type layer 58 comprised of GaAs is formed at the upper layer of the p-type clad layer 56 in the current injection stripe region and the block layer 57.
However, the above self pulsation type laser diode of the prior art has the following problems:
1. The operating current is 56 mA when the output is 5 mV at room temperature of 25xc2x0 C., so the operating current is large.
2. Along with the large operating current, the long term reliability (life) as a laser diode element declines.
3. It is necessary to accurately control the doping amount and layer thickness of the saturatable absorption layer, so there are problems that the degree of self pulsation varies and kink level declines. consequently, the defect rate becomes high in mass production.
4. Two or three epitaxial growth steps are necessary in the production process, so the process becomes complicated.
Also, in recent years, for example, a compatible optical pickup enabling playback of CDs by a DVD optical disk apparatus has been developed. A dual-wavelength monolithic laser diode has been developed wherein for example a laser diode for CD (emission wavelength of 780 nm) and a laser diode for DVD (emission wavelength of 650 nm) are mounted on one chip. Although a laser diode element on the 650 nm side of the dual-wavelength laser as such is desired to be self pulsation type, when trying to incorporate a self pulsation type laser diode of the prior art, three or four epitaxial growth steps are necessary together with the laser diodes on the 780 nm band side and 650 nm band side, so there is a problem that the process becomes long and complicated and the defect rate becomes high in mass production.
An object of the present invention is to provide a self pulsation type laser diode capable of improving long term reliability by reducing the operating current and able to be produced by less number of epitaxial steps at an improved production yield and a semiconductor light emitting apparatus having a plurality of laser diode elements including the laser diode.
According to a first aspect of the present invention, there is provided a laser diode comprising a first clad layer of a first conductivity type formed on a substrate; an active layer formed at an upper layer of the first clad layer; a second clad layer of a second conductivity type formed at an upper layer of the active layer; a third clad layer of the second conductivity type formed at an upper layer of the second clad layer in a current injection stripe region; a contact layer formed at an upper layer of the third clad layer; and an electrode formed so as to connect the second clad layer in regions other than the current injection stripe region and to connect the contact layer; whereby when a first current is injected from the electrode via the contact layer by applying a predetermined voltage to the electrode and laser light is emitted from a laser light oscillation region near the active layer, a second current which is smaller than the first current is injected in regions other than the current injection stripe region from the electrode via the second clad layer and currents at ends of the laser light oscillation region are controlled for self pulsation.
Preferably, in the laser diode according to the present invention, saturatable absorption regions are formed at the ends of the laser light oscillation region for self pulsation.
Preferably, in the above laser diode according to the present invention, the second clad layer comprises an AlGaInP-based material.
Preferably, in the above laser diode according to the present invention, the material of the electrode at a portion contacting the second clad layer comprises titanium.
Further preferably, the electrode comprises stacked layers of titanium, platinum, and gold and formed so as to contact the second clad layer and contact layer from its titanium side.
Preferably, the above laser diode according to the present invention comprises an etching stop layer between the second clad layer and the third clad layer.
Preferably, in the above laser diode according to the present invention, the degree of self pulsation can be adjusted by a thickness of the third clad layer and a width of the current injection stripe region, and more preferably, the thickness of the third clad layer is in a range of 0.1 to 0.7 xcexcm, or, the width of the current injection stripe region is in a range of 1.5 to 5 xcexcm.
In the above configuration of the laser diode of the present invention, it was found that at the time of injecting a first current from an electrode via the above contact layer and emitting laser light from a laser light oscillation region near the active layer, a second current which is smaller than the current is injected from the electrode via the second clad layer in the regions except for the current injection stripe region, so that the current is suitably dispersed to the end portions of the laser light oscillation region and self pulsation occurs.
Furthermore, the configuration and conditions of a laser diode for controlling the strength of self pulsation, kink, and emission angle characteristics were found.
A self pulsation type laser diode having the above configuration is capable of reducing an operating current from that in the prior art. As a result, long term reliability can be improved. Also, only one epitaxial step is necessary due to the configuration, so the number of epitaxial steps can be reduced from that in the prior art and the production yield can be improved.
According to a second aspect of the present invention, there is provided a semiconductor light emitting device comprising a plurality of laser diode elements, wherein at least one of the laser diode elements comprises: a first clad layer of a first conductivity type formed on a substrate; an active layer formed at an upper layer of the first clad layer; a second clad layer of a second conductivity type formed at an upper layer of the active layer; a third clad layer of the second conductivity type formed at an upper layer of the second clad layer in a current injection stripe region; a contact layer formed at an upper layer of the third clad layer; and an electrode formed so as to connect the second clad layer in regions other than the current injection stripe region and to connect the contact layer; and whereby at least the one of the laser diode elements is a laser diode wherein, when a first current is injected from the electrode via the contact layer by applying a predetermined voltage to the electrode and a laser light is emitted from a laser light oscillation region near the active layer, a second current which is smaller than the first current is injected in regions other than the current injection stripe region from the electrode via the second clad layer and currents at ends of the laser light oscillation region are controlled for self pulsation.
Preferably, the plurality of laser diode elements are formed on the same substrate.
According to the above semiconductor light emitting device, for example, in a dual-wavelength monolithic laser diode wherein a laser diode for CDs (light emitting wavelength of 780 nm) and a laser diode for DVDs (light emitting wavelength of 650 nm) are mounted on one chip, a laser diode of a self pulsation type can be incorporated which has excellent noise characteristics capable of reducing the operating current from that in the prior art whereby long term reliability is improved. Furthermore, the number of epitaxial steps can be reduced from that in the prior art and the production yield can be improved.
According to a third aspect of the present invention, there is provided a method of producing a laser diode, including the steps of forming a first clad layer of a first conductivity type on a substrate; forming an active layer at an upper layer of the first clad layer; forming a second clad layer of a second conductivity type at an upper layer of the active layer; forming a third clad layer of the second conductivity type at an upper layer of the second clad layer; forming a contact layer at an upper layer of the third clad layer; forming a mask layer for protecting a current injection stripe region; removing the third clad layer and the contact layer while leaving the current injection stripe region by using the mask layer as a mask; and forming an electrode so as to connect to the second clad layer in regions other than the current injection stripe region and to cennect the contact layer.
Preferably, a method of producing a laser diode according to the present invention further includes the step of forming an etching stop layer at an upper layer of the second clad layer between the step of forming the second clad layer and the step of forming the third clad layer; wherein the third clad layer is formed at an upper layer of the etching stop layer in the step of forming the third clad layer; and the third clad layer and the contact layer are removed by using the etching stop layer as an etching stop, and furthermore, etching conditions are changed for removing the etching stop layer in the step of removing the third clad layer and the contact layer.
Preferably, in a method of producing a laser diode according to the present invention, titanium is used as a material of the electrode of a portion contacting the second clad layer in the step of forming the electrode.
Further preferably, stacked layers of titanium, platinum, and gold are formed as the above electrode so as to contact the second clad layer and contact layer from its titanium side in the step of forming the electrode.
Preferably, in a method of producing a laser diode according to the present invention, a thickness of the third clad layer is formed to be in a range of 0.1 to 0.7 xcexcm.
Also preferably, a width of the current injection stripe region is formed to be in a range of 1.5 to 5 xcexcm.
According to the above method of production of a laser diode of the present invention, a laser diode of a self pulsation type having excellent noise characteristics capable of reducing an operating current from that in the prior art whereby long term reliability is improved can be produced by less number of epitaxial steps than in the prior art at an improved production yield.